1. Field of the Invention
Embodiments of the invention relate to non-volatile memories. More particularly, embodiments of the invention relate to a bi-directional resistive random access memory (RRAM) capable of multi-decoding and a method of writing data to the bi-directional RRAM.
2. Discussion of Related Art
Next generation memory demands require highly integrated dynamic random access memory (DRAM), non-volatile flash memory and high-speed static random access memory (SRAM) devices. Currently, phase-change random access memory (PRAM), nano-floating gate memory (NFGM), polymer random access memory (PoRAM), magnetic random access memory (MRAM), ferro-electric random access memory (FRAM), RRAM or the like are regarded as the next generation memory devices that meet these demands.
FIG. 1 is a schematic view of a cell C structure of a conventional bi-directional RRAM having a non-ohmic NO device and a resistance variable device RV. In the RRAM, data is written using variations in the resistance-value of the device. The resistance variable device includes a resistance variable substance disposed between first and second electrodes. The resistance value varies in accordance with an applied voltage or an applied current. For example, in a uni-directional RRAM, the resistance value varies in accordance with the amount of applied voltage or applied current. In a bi-directional RRAM, the resistance value varies in accordance with the amount and the direction of the applied voltage or applied current.
The bi-directional RRAM illustrated in FIG. 1 realizes bi-directivity by including a non-ohmic device. The non-ohmic device is in a high-resistance state when the applied voltage is within a predetermined voltage range of −3V to ˜3V with no current being applied. The non-ohmic device is in a low-resistance state when the applied voltage is outside of the predetermined voltage range of −3V to ˜3V with an applied current. An example of a bi-directional RRAM utilizing a non-ohmic device and a resistance variable device is disclosed in U.S. Pat. No. 6,909,632.
FIG. 2 is a graph illustrating cell characteristics of the conventional bi-directional RRAM illustrated in FIG. 1. When a writing voltage VW of 6V is applied to the resistance variable substance, a corresponding cell has a first resistance. When a writing voltage −VW of −6V is applied to the resistance variable substance, a corresponding cell has a second resistance. A data value “1” can correspond to when a cell has the first resistance and a data value “0” can correspond to when a cell has the second resistance. That is, in the bi-directional RRAM, the data values “1” and “0” can be written using writing voltages VW and −VW where the magnitudes of these writing voltages at both ends of the cell are the same but the polarities are different.
FIGS. 3A and 3B are schematic views illustrating the operation of writing data to a cell of the conventional bi-directional RRAM illustrated in FIG. 1. Referring first to FIG. 3A, a data value “0” is written to a cell (indicated by a circle) by applying 3V to a word line WL and −3V to a bit line BL (WRITE 0). A data value “1” is written to the cell by applying −3V to the word line WL and 3V to the bit line BL (WRITE 1). Here, 0V is respectively applied to an unselected word line WL′ and an unselected bit line BL′. Referring to FIG. 3B, a data value “0” is written to a cell by applying 6V to a word line WL, 0V to a bit line BL, and 3V respectively to an unselected word line WL' and an unselected bit line BL' (WRITE 0). On the other hand, a data value “1” is written to the cell by applying 0V to the word line WL, −6V to the bit line BL, and −3V respectively to the unselected word line WL' and the unselected bit line BL' (WRITE 1). The arrows of FIGS. 3A and 3B represent the flowing of the current during the period of writing a data value 0 (WRITE 0) or 1 (WRITE 1).
However, when data is written by applying the writing voltages VW and −VW (VW=6V, −VW=−6V) to a word line WL or a bit line BL as shown in FIG. 3B, the voltage of an unselected word line WL′ and an unselected bit line BL′ is changed in accordance with a data value. Thus, it is more efficient to write the data by applying ½ writing voltages ½VW and −½VW (½VW=3V, −½VW=−3V) to the word line WL or the bit line BL as shown in FIG. 3A. As memory capacity demands increase, multi-decoding of a bi-directional RRAM is required to perform addressing at high speeds and to reduce the resulting chip size.